This invention relates to a refrigerant circulating system which uses a refrigerant which essentially contains hydrofluorocarbon or third generation refrigerants.
An example of a conventional refrigerant circulating system is as shown in FIG. 14.
As indicated by the publication "Tribologist" Vol. 35, No. 9 (1990) pp. 621 to 626, in forming a refrigerating machine using an HFC134a refrigerant containing hydrofluorocarbon, PAG (polyether) or ester refrigerating machine oil is employed, because the solubility of the refrigerant and the refrigerating machine oil is one of the important factors determining the performance of the refrigerant circulating system. The conventional refrigerant circulating system shown in FIG. 14 is a refrigerating machine using an HFC134a refrigerant. In FIG. 14, reference numeral 1 designates a compressor for compressing a refrigerant gas; 2, a condenser for condensing a high pressure refrigerant gas discharged from the compressor 1; 3, a throttle mechanism; 4, an evaporator; 5, a 4-way valve having a function of reversing the direction of flow of the refrigerant; 8, an accumulator adapted to adjust the quantity of refrigerant; and 6a, 6b a refrigerating machine oil kept in the compressor 1 to lubricate the slide parts of the compressor 1 and to seal the compressing chamber thereof, the oil being PAG 6a or ester refrigerating machine oil 6b.
The behavior of the oil in the system will be described. The refrigerant compressed by the compressor 1 is supplied into the condenser 2. A larger part of the lubricant 6a or 6b used for lubricating the compressor and sealing the compressing chamber returns to the bottom of the compressor; however, the lubricant 6a or 6b which corresponds to about 0.5 to 3.0% of the refrigerant by weight is discharged together with the refrigerant from the compressor. The oil 6a or 6b thus discharged is soluble with the refrigerant. Therefore, it flows smoothly through the condenser 2, the capillary tube 3, and the evaporator 4 into the accumulator 8, where it is dissolved in the excess of liquid refrigerant 7a, thus returning through an oil returning hole 82 into the compressor 1. Hence, the lubricant 6 is kept in the compressor 1 at all times, thus performing the lubrication satisfactorily.
Another example of the conventional refrigerant circulating system is as shown in FIG. 15.
Heretofore, as disclosed by Japanese Patent Unexamined Publication (Kokai) Hei-5-17789/(1992), a refrigerant circulating system employing a refrigerant mixture containing an inflammable refrigerant HFC32 uses a refrigerating machine oil such as ester refrigerating machine oil high in solubility. FIG. 15 shows an air conditioner using a refrigerant mixture of HFC32, HFC125 and HFC134a. In FIG. 15, reference numeral 1 designates a compressor for compressing the refrigerant gas; 2, a condenser for condensing a high pressure refrigerant discharged from the compressor 1; 3, a throttling mechanism; 4, an evaporator; 5, a 4-way valve for reversing the flow of refrigerant; 8, an accumulator having a function of adjusting the quantity of refrigerant; and 6b, a refrigerating machine oil used for lubricating the slide parts of the compressor 1 and sealing a compression chamber. More specifically, the refrigerating machine oil is an ester refrigerating machine oil 6b.
The refrigerant sucked into the compressor 1, after cooling an electric motor 12 in a closed container 11, is compressed by a compressing mechanism 13, and discharged through the 4-way valve 5 into the condenser 2. In this operation, the refrigerating machine oil 6b pooled in the bottom of the closed container 11 is exposed to the atmosphere of the refrigerant sucked into the compressor, and a large quantity of refrigerant is dissolved into the refrigerating machine oil 6b, because the latter 6b is high in solubility.
The conventional HFC134a refrigerant circulating system is designed as described above. The PAG (polyether) 6a has a volume resistivity of 10.sup.7 to 10.sup.10 .OMEGA.cm, and a saturated water content of about 25000 ppM; whereas the ester refrigerating machine oil, 6b is superior in characteristic to the PAG, having a volume resistivity of 10.sup.12 to 10.sup.14 .OMEGA.cm, and a saturated water content of about 1500 ppM. However, those oils are much inferior in electrical insulation and hygroscopicity to a CFC12 refrigerating machine oil which is 10.sup.15 .OMEGA.cm in volume resistivity and about 300 ppM in saturated water content.
Hydrofluorocarbons (HFC) are promising refrigerants that could replace CFC12 and HCFC22 which are on the list of materials that need be used less or totally disused in view of the environmental problems they have caused. Hydrofluorocarbons generally are more polar than CFC12 and HCFC22 and refrigerating oils that are highly soluble with such hydrofluorocarbons are also generally high in polarity. Hence, those oils which are soluble with hydrofluorocarbons tend to absorb moisture having high polarity. As a result, moisture will be carried over into the refrigerant circulation system and there is high likelihood for deterioration not only in electrical insulation but also in hydrolyzable materials such as refrigerating ester oils and those organic materials which are used in the compressor. If the deteriorated materials accumulate in the throttle mechanism, the performance of the refrigerant circulation system can potentially drop.
Hence, in assembling the refrigerant circulating system, it is necessary to shorten the period of time as much as possible for which the system is left open in the air. That is, the production of the refrigerant circulating system suffers from a number of problems in handling the components. Furthermore, the system involves the following problems: If a large quantity of water content enters the system, the formation of sludge is accelerated, or the water content is freezed to close the throttle mechanism, thus impeding the cooling operation.
In addition, the conventional HFC134a refrigerant circulating system is disadvantageous in the following points: In servicing the system for instance by repairing or replacing the components of the system, because of the high hygroscopicity of the refrigerating machine oil the water content of air is adsorbed on the refrigerating machine oil, so that the capillary tube or the expansion valve of the throttle mechanism is liable to be closed being freezed. In addition, the water content accelerates the hydrolysis of the ester oil, thus forming sludge. Furthermore, the water content accelerates the hydrolysis of polyethylene terephthalate employed as the insulating material of the motor, thus forming sludge. In order to eliminate the above-described difficulties accompanying the manufacture of the refrigerant circulating system or the service on it, it is necessary to more sufficiently remove the water content from the refrigerating machine oil and from the refrigerant cycling system than in the system using a CFC12 refrigerant. In addition, it is necessary to provide a drier in the refrigerant cycling system so as to complement the water content.
If a refrigerant is used together with a highly soluble refrigerating oil, the amount of the refrigerant that is dissolved in the refrigerating oil will increase rapidly as the temperature of the oil or the refrigerant in contact with it approaches the saturation point of the latter at the pressure of the atmosphere in which the refrigerating oil is placed. Since the refrigerating oil is diluted with a less viscous refrigerant, the above phenomenon will cause an extreme drop in the viscosity of the liquid being supplied to the sliding part and its lubrication characteristics will deteriorate to potentially cause seizure and other problems.
In the case where the bottom of the compressor to contain a highly soluble refrigerating oil is located on the high-pressure side, the solubility of the refrigerant will increase in the high-pressure atmosphere in the compressor where the areal pressure at the sliding part increases to require better lubrication characteristics and, hence, the viscosity of the liquid being supplied to the sliding part will drop and its lubrication characteristics will deteriorate to potentially cause seizure and other problems. Therefore, in the case under consideration (i.e., the bottom of the compressor to contain a highly soluble refrigerating oil is located on the high-pressure side), it has been common practice to use a viscous refrigerating oil in view of its dissolution under high pressure but this presents a problem in a low-pressure atmosphere. That is, the liquid to be supplied to the sliding part becomes extremely viscous due to small dissolution of the refrigerant and the mechanical loss caused by shear stress in the oil at the slide bearing or the like increases in its ratio to work of compression, leading to a lower efficiency.
Speaking also of the case of using a highly soluble refrigerating oil, the part of the oil which leaks out of the oil seal portion towards the suction side of the compression space contains a large amount of refrigerant, which will be expanded and compressed again in the compression cycle, causing loss and subsequent decrease in efficiency.
In the case where, in the conventional refrigerant circulating system, a refrigerant mixture containing at least an inflammable refrigerant is employed; more specifically, a refrigerant mixture containing an inflammable refrigerant HFC32 and nonflammable refrigerants HFC125 and HFC134a, is employed, large quantities of the refrigerants of the refrigerant mixture are dissolved in the refrigerating machine oil, because the latter is high in solubility. In this case, the quantities of the refrigerants dissolved in the refrigerating machine oil depend on the polarities of them. FIG. 16 indicates the solubilities of HFC32, HFC125 and HFC134a with respect to ester refrigerating machine oil. The refrigerants HFC125 and HFC134a relatively low in polarity are dissolved in the refrigerating machine oil more than the refrigerant HFC32 relatively high in polarity. As a result, the mixing ratio of the refrigerant mixture in the refrigerant circulating cycle which is not dissolved in the refrigerating machine oil yet, may change into a mixing ratio with which it is inflammable, because the rate of the refrigerant HFC32 which is inflammable when isolated is increased with respect to the mixing ratio which it had before being put in the refrigerant circulating cycle and was detected nonflammable. Hence, if the refrigerant mixture is discharged into the air by accident, and an igniting source is present near it, then it is ignited, and it may cause a fire.
A conventional refrigerant circulating system using a refrigerant mixture containing a plurality of refrigerants at least one of which is inflammable, employs an inflammable refrigerating machine oil. Hence, the system suffers from the following problem: When, because of some failure, the refrigerant mixture and the refrigerating machine oil are discharged in the form of mist into the air, and there is an igniting source near them, they may be ignited; that is, a fire may be started.
The term "inflammable refrigerant" as used herein is intended to mean the refrigerant which, upon touching an igniting source, is ignited if its temperature is in a practical temperature range, and its mixing ratio with air is a predetermined value. Examples of the inflammable refrigerant are, for instance, HFC32 (R32), HFC143a (R143a), HFC152a (R152a), propane (R290), butane (R600), pentane, and ammonia (R717). On the other hand, the term "nonflammable refrigerant" as used herein is intended to mean the refrigerant which is never ignited by an igniting source irrespective of its mixing ratio with air if its temperature is in a practical temperature range. In the above-described definitions, the practical temperature range is from -40.degree. C. to +100.degree. C.